Minnesota is warming, and faster than most of the rest of the country.

Since 1970, the state famed for its deep-freeze sensibilities was the nation's third-fastest-warming state, and its average minimum temperatures rose faster than any other state's, according to a study by Climate Central, a nonprofit funded by NASA, NOAA, the National Institutes of Health and several private foundations.

"I was buying an air conditioner in March -- and they were sold out," said assistant state climatologist Pete Boulay, recalling a recent record warm stretch that wasn't included in the Climate Central study.

Claudia Tebaldi, a coauthor of the study, said in an e-mail that the causes of Minnesota's warming are the same as those across much of the Northern Hemisphere: increasing concentrations of greenhouse gases in the atmosphere, along with local natural variations.

One key variation in Minnesota, Boulay said, would be the reduction of consistent deep snow cover, resulting in darker, more radiation-absorbing ground during the winter.

The Climate Central study, using readily available daily temperature data, looked at two periods: from 1912-2011 and 1970-2011. It determined that Minnesota's average daily temperature rose 2.5 degrees since 1970, compared with 1.7 degrees nationally. From 1912 to 2011, Minnesota's average increased 2.3 degrees, ninth among the states and well ahead of the national rise of 1.3 degrees.

In both the 100- and 40-year periods, the states with the steepest increases in average temperatures were across the northern tier and in the Southwest (Wisconsin was fourth, right behind Minnesota, from 1970 to 2011). Three Southeastern states actually cooled over the past century, although they, like all other states, warmed over the more recent 40-year period.

Changes seen in Minnesota

The impacts of Minnesota's warming, particularly in winter and spring, have been well-documented in recent years: earlier ice-outs on lakes, earlier spring planting and emergence, warming of Lake Superior, the apparently northward creep of some hardwood tree species and the cancellation or postponement of major winter celebrations -- not to mention lower heating bills.

Many climate experts also note a link between increased warmth and a steady increase in annual precipitation across Minnesota, because warmer air can hold more moisture.

John Edman, director of Explore Minnesota, noted that the state's winter tourism industry has had to adapt -- not to a state without winter, but to one where winter is less reliable. That has meant plugging into arts and culture and other activities less weather-related than skiing and snowmobiling.

'Conversation starter'

Tebaldi described the study as "a conversation starter," intended to create a more targeted type of awareness of climate change, which is usually cast in global terms.

The study says that some areas may not have warmed as quickly as others because air pollution might be protecting them from radiation from the sun. In a twist, the study noted that clean-air standards have dramatically reduced emissions, which could lessen such protection.

In her e-mail, Tebaldi said that "local variability may still have a buffering effect [on warming], and it is important to realize climate change is not going to be uniform and steady everywhere. But we should not rely on these natural variations (and even less on noxious air pollution!) to protect us indefinitely."

Recent experience in Minnesota, outside the study period, has continued the warm pattern. Meteorological spring in the Twin Cities was the warmest on record -- though it was for the entire United States as well. Minnesota had its warmest March ever, and May was the 12th-straight month with an average temperature above the 30-year normal. Boulay said he knows of no other such stretch on the books.

Bill McAuliffe • 612-673-7646